There is a desire for low cost electrical machines for converting between electrical and mechanical energy that have both high efficiency and high performance capability. Unfortunately, current devices tend to suffer from one of more of a number of deficiencies that include low efficiency, low power density, and complex and expensive constructions.
Conventional permanent magnet radial gap motor/generators utilize a rotor with banded permanent magnets that is surrounded by a laminated stator. The rotor has a limited magnet peripheral speed and hence the power capability per amount of magnet and total weight is less than optimal. The magnets must also drive magnetic flux through a high-strength reinforcing band located in the magnetic airgap, which further reduces the power capability. The magnetic flux passing through a laminated stator incurs magnetic hysteresis and eddy current losses, which reduce efficiency.
Axial gap electrical machines can increase the magnet peripheral speed and power capability for a given weight and rotational speed because the magnets can be located at a larger diameter. This is at the expense of a larger diameter motor/generator rotor, which can be acceptable in many applications. Air core windings can also be utilized instead of slot windings to reduce magnetic losses. Unfortunately axial gap motor/generators encounter several problems, including complex construction, required subassembly machining, very high stresses, expensive and thick composite material reinforcement bands, low magnet strength issues, temperature limitations and high costs.
A prior art rotor for a brushless, axial gap, air core electrical machine is shown in FIG. 1A. The rotor 30 is comprised of a circumferential array of axially magnetized permanent magnets 31 that are connected to a central hub 32. The magnets are banded with a stainless steel band 33 having an optimized radial thickness to minimize the stress in the band 33. The stresses in the stainless steel band are shown in FIG. 1B. When rotating to a magnet peripheral speed of 265 m/sec, the radial stress remains low. However, the hoop stress far exceeds the allowable stress for the stainless steel band 33. The rotor speed must be substantially reduced for safety, reducing the power capability.
A second configuration prior art rotor for a brushless, axial gap, air core electrical machine is shown in FIG. 2A. The rotor 40 is comprised of a circumferential array of multiple axially magnetized permanent magnets 41 that are connected to a central hub 42. The magnets are reinforced or preloaded by an outer carbon fiber epoxy band 43 with radial thickness optimized to minimize stress. The stresses in the band 43 are shown in FIG. 2B. The radial stress remains low when rotated to the same 265 m/sec operating speed. However, the hoop stress is high, close to its allowable stress level. The design is therefore not very robust and desirable. Composite material bands typically suffer from low maximum temperature performance. If the rotor temperature becomes elevated, the strength will reduce and the rotor will no longer be able to safely operate at that speed. The composite band is expensive and also has a very low coefficient of thermal expansion, making the assembly process difficult. Furthermore, the large radial thickness of the composite band negatively imparts the electrical machine armature winding configuration and overall performance. A new type of axial gap brushless motor/generator that has improved performance as well as low cost and reliable construction is needed.